Hybrid electric vehicles are widely viewed as being among the most practical of the low-polluting vehicles. A hybrid electric vehicle includes an electric "traction" battery which provides electric power for an electric traction motor, which in turn drives the wheels of the vehicle. The "hybrid" aspect of a hybrid electric vehicle lies in the use of a secondary or supplemental source of electrical energy for recharging the traction battery during operation of the vehicle. This secondary source of electrical energy may be solar panels, a fuel cell, or a generator driven by an internal combustion engine. When an internal combustion engine is used as the secondary source of electrical power, it commonly is a relatively small engine which uses little fuel, and produces little pollution. A concomitant advantage is that such a small internal combustion engine can be operated within a limited RPM range, so that pollution controls of the engine may be optimized.
The terms "primary" and "secondary" when used to describe the sources of electrical energy merely relate to the way energy is distributed during operation, and are not of fundamental importance to the invention.
The traction battery of a vehicle is often made up from a plurality of series-connected modules, each of which in turn is made up from one or more interconnected battery cells. One traction battery which is contemplated for use with the invention has a nominal voltage of 550 volts, and is made up of forty-six series-connected 12-volt modules. Each module may, of course, be made up of one or more common automobile-type batteries. A hybrid electric vehicle, unlike a purely electrically powered vehicle, is at least partially recharged during operation; that is to say, that the traction battery ordinarily does not arrive at a depot in a substantially discharged state, and consequently will not be in dire need of recharging. As a result, a hybrid electric vehicle is used much as an ordinary internal-combustion driven vehicle; it can be parked after a day's use, and started in the morning without recharging.
The most commonly available rechargeable batteries are the ordinary automotive-style lead-acid batteries, which use porous lead or lead composition plates, and use a dilute sulfuric acid electrolyte. It is desirable for reasons of cost and availability to use such batteries for hybrid electric vehicles. One disadvantage of lead-acid batteries is their weight, and another is the reduction of the electric charge capacity of the battery with use. The reduction of charge capacity manifests itself as an increase of impedance, which in turn reduces the current which can be drawn from the battery at any given voltage. The reduced charge capacity arises, at least in part, from "sulfation" or formation of a sulfate coating on the plates of the battery. This sulfation tends to occur when the battery is allowed to remain for long periods of time in a partially charged condition. Thus, when a hybrid electric vehicle is repeatedly parked overnight, sulfation tends to occur, with the result of loss of traction battery charge capacity.
Improved methods for maintenance of the batteries of hybrid electric vehicles are desired.